An optical reflection modulator is known from German Patent Publication DE 40 31 970. Here, the light emerging from the blunt end of an optical fiber strikes a mirror disposed orthogonally to the fiber. This mirror reflects the light back onto itself, creating a standing wave in the manner of a Fabry-Perot resonator between the reflecting fiber end and the mirror, provided their distance from each other corresponds to a multiple of half a wavelength of the light used. By changing this distance, particularly by displacing the mirror, the Fabry-Perot resonator can be detuned and the intensity of the light can be changed.
The described modulator is of a relatively simple design and has good modulating properties. However, it is dependent on the absolute value of the distance between the fiber end and the mirror, which changes because of many effects, for example, as a function of the temperature. It is therefore necessary in such arrangements to provide a regulator which maintains the effective path length or "optical distance" between the fiber end and the mirror constant.